High strength nickel-base alloys



United States Patent 3,351,463 HIGH STRENGTH NICKEL-BASE ALLOYS Alexander G. Rozner and William J. Buehier, Bethesda,

Md., assignors to the United States of America as represented by the Secretary of the Navy No Drawing. Filed Aug. 20, 1965, Ser. No. 481,436 10 Claims. (Cl. 75170) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention pertains to high strength nickel based alloys and more particularly to high strength near stoichiometric nickel-titanium alloys and a method for their preparation.

The near stoichiometric nickel-titanium alloys and their unique properties are fully described in Patent No. 3,174,851. For many applications, however, it would be desirable to produce these alloys with higher yield strengths but attempts to effect increases in yield strengths by conventional means such as heat treatment have met with little success, probably due to the fact that the alloy exists essentially in a single phase.

Accordingly, it is an object of this invention to produce high strength near stoichiometric nickel-titanium alloys.

It is another object of this invention to provide a novel method for producing such high strength alloys.

It is a further object to provide nickel-titanium alloys with improved yield strengths without significantly afiecting ductility and toughness.

The objects of this invention are accomplished by producing a nickel-titanium alloy having a yield strength greater than 100,000 p.s.i., an elongation greater than 7%, an ultimate tensile strength greater than 160,000 p.s.i., a strength to density ratio greater than 400x10 inches and a magnetic permeability of essentially unity, by cold working a near stoichiometric nickel-titanium alloy 53.55 6.5 weight percent nickel with the remainder being essentially titanium and hereinafter called TiNi). The invention will be more fully described in the light of the following background information.

The process of this invention is based upon a finding that working TiNi will produce a martensitic (diffusionless) transition. The ability of the alloy to undergo a martensitic transition is temperature dependent with the maximum temperature at which this transition can occur being called the critical temperature. The critical temperature, which is a function of the alloy composition, may be readily determined from a damping transition curve and some typical compositions with their approximate critical temperatures appear in Table I.

Table I Alloy composition-Wt. Critical percent Ni: temp. C. 53.5 98 54.0 140 54.5 170 55.0 140 55.5 30 56.0 -25 56.5 50

Thus, in particular the process of this invention for strengthening TiNi comprises working the alloy below its critical temperature, i.e., at any temperature at which the alloy will undergo a martensitic transition. The strengthening time is temperature dependent, with the greater the temperature gradient between the working temperature and critical temperature, the faster the strengthening. For most purposes, it is preferred that the working temperature be maintained at least 20 or 30 C. below the critical temperature and in order to insure that the minimum desired temperature gradient is maintained, the temperature rise ordinarily caused by friction must be taken into account. It should be remembered, however, that as long as the working temperature is kept below the critical temperature, temperature variations caused by friction will not significantly affect the process.

The alloy may be worked by any one of the well known means currently used, from among which there can be mentioned swaging, rolling, drawing, extruding, forging, explosive forming, stretch working, roller leveling, etc., said working preferably being performed as part of the general procedure for fabricating the alloy into a finished structure.

The amount of work done on the alloy will be dependent upon the strength and hardness desired, with greater working producing greater strength. It should be apparent, however, that there is an upper limit to the amount of work that the alloy can be subject to and if it is exceeded, edge cracking and local explosion may occur. As a rule of thumb, it has been determined that the alloy should not be Worked beyond a 20 or 25 percent reduction in area for round bars or a 20 or 25 percent reduction in thickness for flat bars since this excess working results in a significant decrease in ductility without a comparable increase in strength.

The process of this invention produces a novel alloy which combines very high strength, high hardness, high impact resistance (toughness), good tensile elongation (ductility), corrosion resistance, lower density and nonmagnetic stability. The alloy of this invention preferably has a yield strength between about 120,000 and 200,000 p.s.i., a total elongation between about 7% and 20%, a strength to density ratio between about 520 10 and 800x10 inches, an ultimate tensile strength between 190,000 and 260,000 psi. and a magnetic permeability of essentially unity.

The following examples illustrate a specific embodiment of the invention but they are not to be considered as limiting the invention in any manner.

EXAMPLE I Table II a Yield Strength Ultimate Total Reduction in Thickness (lbs/111. Tensile Elongation (Percent) (0.2% onset) Strength (Percent) (lbs/in 0 (annealed) 30X10 130 l0 22 10 163 25 15 191 19 In addition to the high strength and ductility as measured by yield strength and elongation respectively, the alloy had a magnetic permeability of essentially unity and high corrosion and impact resistance.

EXAMPLE II A .06 inch diameter TiNi (55.1 weight percent nickel) wire was annealed and cooled as in Example I. The alloy was then drawn through conical dyes and the amount of work done (as measured by reduction in area) with the corresponding properties are tabulated below.

The alloy had a magnetic permeability of essentially unity and a high impact and corrosion resistance.

The alloys of this invention have many structural applications since they combine the highly desirable properties of strength, toughness, ductility, corrosion resistance and magnetic permeability of essentially unity. The high strength to density ratio makes these alloys especially valuable for applications where strong lightweight materials are essential, as for example, rocket cases and space components. The high strength and corrosion resistance of these alloys (particularly in marine media) make them suitable for low noise ship propellers in spite of the fact that some of their vibration damping ability is sacrificed for improved strength, The alloy will also find use in armour-like materials for resisting ballistic impact.

The process of this invention may be performed by now existing working procedures and has the advantages that unlike conventional slip and dislocation work hardening processes it does not significantly affect ductility and toughness and unlike the austentite to martensite transition in steel it does not affect the non-magnetic properties of the alloy.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An alloy comprising 5 3.55 6.5 weight percent nickel the remainder being essentially titanium, said alloy having a yield strength greater than 100,000 p.s.i., an elongation greater than 7%, an ultimate tensile strength greater than 160,000 p.s.i., a strength to density ratio greater than 400x10 inches and a magnetic permeability of essentially unity.

2. An alloy comprising 53.5-56.5 weight percent nickel the remainder being essentially titanium, said alloy having a yield strength between about 120,000 p.s.i. and 200,000 p.s.i., an elongation between about 7% and 20%, an ultimate tensile strength between about 190,000 p.s.i. and 260,000 p.s.i., a strength to density ratio between about 520x10 and 800 10 inches and a magnetic permeability of essentially unity.

3. A process for treating an alloy comprising 53.5 56.5 weight percent nickel the remainder being essentially titanium which comprises working said alloy below its critical temperature in order to increase its yield strength.

4. The process of claim 3 wherein said temperature is at least 20 C. below the critical temperature.

5. The process of claim 3 wherein said alloy comprises about 55.1 weight percent nickel the remainder essentially titanium.

6. The process of claim 3 wherein said temperature is at least 30 C. below the critical temperature.

7. The process of claim 3 wherein the working is accomplished by rolling.

8. The process of claim 3 wherein the alloy is in the form of a wire and the working is accomplished by draw- 9. A process for treating an alloy comprising 55.1 weight percent nickel the remainder essentially titanium which comprises working said alloy at a temperature that is at least 20 C. below the alloys critical temperature in order to increase its yield strength.

10. The process of claim 9 wherein said temperature is at least 30 C., below the critical temperature.

References Cited UNITED STATES PATENTS 3,174,851 3/1965 Buehler et al. 170

DAVID L. RECK, Primary Examiner.

RICHARD O. DEAN, Examiner. 

1. AN ALLOY COMPRISING 53.5-56.5 WEIGHT PERCENT NICKEL THE REMAINDER BEING ESSENTIALLY TITANIUM, SAID ALLOY HAVING A YIELD STRENGTH GREATER THAN 100,000 P.S.I. AN ELONGATION GREATER THAN 7%, AN ULTIMATE TENSILE STRENGTH GREATER THAN 160,000 P.S.I., A STRENGTH TO DENSITY RATIO GREATER THAN 400X10**3 INCHES AND A MAGNETIC PERMEABILITY OF ESSENTIALLY UNITY. 